Ferrite is a generic term for any compound including an oxide of a divalent cationic metal and trivalent iron, and ferrite magnets have found a wide variety of applications in numerous types of rotating machines, loudspeakers, and so on. Typical materials for a ferrite magnet include Sr ferrites (SrFe12O19) and Ba ferrites (BaFe12O19) having a hexagonal magnetoplumbite structure. Each of these ferrites is made of iron oxide and a carbonate of strontium (Sr), barium (Ba) or any other suitable element, and can be produced at a relatively low cost by a powder metallurgical process.
A basic composition of an (M-type) ferrite having the magnetoplumbite structure is normally represented by the chemical formula AO.6Fe2O3, where A is a metal element to be divalent cations and is selected from the group consisting of Sr, Ba, Pb, Ca, and other suitable elements.
It was reported that the magnetization of a Ba ferrite could be increased by substituting Ti or Zn for a portion of Fe (see Journal of the Magnetics Society of Japan Vol. 21, No. 2 (1997), 69–72).
It is also known that the coercivity and magnetization of a Ba ferrite can be increased by substituting a rare-earth element such as La for a portion of Ba and by substituting Co or Zn for a portion of Fe (see Journal of Magnetism and Magnetic Materials, Vols. 31–34 (1983), 793–794 and Bull. Acad. Sci. USSR (Transl.) phys. Sec. Vol. 25 (1961) 1405–1408).
As for an Sr ferrite on the other hand, it was reported that the coercivity and magnetization thereof could be increased by substituting La for a portion of Sr (see IEEE Transaction on Magnetics, Vol. 26, No. 3 (1999), 1144–1148).
It was also reported that the coercivity and magnetization of an Sr ferrite could be increased by substituting La for a portion of Sr and by substituting Co and Zn for a portion of Fe (see PCT International Application No. PCT/JP98/00764 (corresponding to PCT International Publication No. WO 98/38654)).
Furthermore, it was reported that a magnet, including, as its main phase, a hexagonal ferrite such as Ba ferrite or Sr ferrite in which Sr, Ba, Ca, Co, rare-earth elements (including Y), Bi and Fe are contained, may be produced by adding some or all of those constituent elements to particles including, as their main phase, a hexagonal ferrite containing at least Sr, Ba or Ca and then calcining the mixture decisively (see PCT International Application No. PCT/JP98/04243 (corresponding to PCT International Publication No. WO 99/16087)). It was reported that a magnet having at least two Curie temperatures could be obtained and the magnetization, coercivity and variation in coercivity with temperature could be improved according to this method.
As for an Sr ferrite or a Ba ferrite, it was further reported that a high-performance ferrite magnet with excellent magnetic properties (in the loop squareness of its B—H curve among other things) could be obtained at a relatively low cost by substituting La, Ce, Pr, Nd, Sm, Eu or Gd for a portion of Sr or Ba and by substituting Co, Mn or V for a portion of Fe (see Japanese Laid-Open Publication No.11-307331).
It was further reported that a ferrite magnet, hardly exhibiting any variation in coercivity or magnetization with temperature (i.e., showing slight decrease in magnetization in a high temperature range and little decrease in coercivity in a low temperature range) could be obtained by allowing a ferrite having an M-type magnetoplumbite structure and a ferrite having a spinel-type structure to coexist (see Japanese Laid-Open Publication No. 11-224812).
However, none of these ferrite magnets can improve the magnetic properties sufficiently and reduce the manufacturing cost significantly at the same time. Specifically, it was reported that the ferrite in which Ti or Zn was substituted for a portion of Fe exhibited slightly increased magnetization but significantly decreased coercivity. It was also reported that the ferrite in which La was substituted for a portion of Sr exhibited slightly increased coercivity and magnetization. However, the properties of such a ferrite are still not fully satisfactory. Furthermore, it was reported that the ferrite in which La was substituted for a portion of Ba or a portion of Sr and in which Co or Zn was substituted for a portion of Fe exhibited increased coercivity and magnetization. But if a rare-earth element (such as La) and Co are used in large amounts as substituents for a ferrite, then the material cost of such a ferrite increases adversely because the raw materials of these substituents are expensive. In that case, the essential feature of the ferrite magnet, which should be produced at a lower manufacturing cost than a rare earth magnet, for example, might be lost. Furthermore, the ferrite in which La, Ce, Pr, Nd, Sm, Eu or Gd was substituted for a portion of Sr or a portion of Ba and in which Co, Mn or V was substituted for a portion of Fe exhibited improved loop squareness but decreased magnetization.
On the other hand, as for the ferrite disclosed in Japanese Laid-Open Publication No. 11-224812, in which a ferrite having an M-type magnetoplumbite structure and a ferrite having a spinel-type structure coexist, ferrites representing Examples Nos. 1 to 3 thereof exhibit improved magnetic properties and temperature properties but include relatively large amounts of La and element M such as Mg, Mn, Cu, Fe, Co, Ni and Li to be added to Fe. It is also disclosed in that document that if a ferrite having a composition represented by Sr0.8La0.2Fe11.8Co0.2O19 is fired within a mixture of nitrogen and oxygen gases with the partial pressure of oxygen changed, a ferrite representing Example No. 4 thereof can have the structure, in which a ferrite having an M-type magnetoplumbite structure and a ferrite having a spinel-type structure coexist, even without using a reducing atmosphere as the firing atmosphere. In that case, however, good magnetic properties are not achieved. Example No. 5 disclosed therein is a ferrite magnet to be obtained, as in the present invention, by separately preparing SrFe12O19 as a ferrite having an M-type magnetoplumbite structure and CoFe2O5 as a ferrite having a spinel-type structure, mixing these ferrites together during a pulverization process, and then making a sintered body by a normal process. In that case, the temperature coefficient of magnetization is improved but the magnetization and coercivity themselves decrease significantly due to the addition of CoFe2O5.
In order to overcome the problems described above, a primary object of the present invention is to provide a ferrite magnet that can be produced at a low manufacturing cost and that can still exhibit improved magnetic properties and a method of making such a ferrite magnet.